1. Field of the Invention
The invention relates to a hybrid vehicle that includes an internal combustion engine and an electric motor as drive sources and that includes a fuel vapor gas purge system for purging fuel vapor produced inside a fuel tank, and a control method for the hybrid vehicle.
2. Description of Related Art
One of hybrid vehicles is able to travel with only the power of an electric motor while the operation of an internal combustion engine is stopped (hereinafter, referred to as “electric travel”) and to travel with both the power of the internal combustion engine and the power of the electric motor (hereinafter, referred to as “hybrid travel”).
More specifically, the hybrid vehicle starts hybrid travel by starting the engine when a vehicle required power that varies with an accelerator operation amount and a vehicle speed becomes larger than or equal to a start power threshold, and performs electric travel by stopping the operation of the engine when the vehicle required power becomes smaller than or equal to a stop power threshold during hybrid travel. That is, the hybrid vehicle operates the engine when the engine can be efficiently operated and/or when sufficient vehicle driving force is not obtained from only the electric motor, and stops the operation of the engine when the engine cannot be efficiently operated and/or when sufficient vehicle driving force is obtained from only the electric motor (for example, see Japanese Patent Application Publication No. 2006-9668 (JP 2006-9668 A)). In this way, the hybrid vehicle intermittently operates the engine. That is, the hybrid vehicle executes intermittent operation of the engine.
On the other hand, the hybrid vehicle includes a chargeable electrical storage device (for example, battery) that is able to supply electric power to the electric motor. Furthermore, in recent years, there has been developed a hybrid vehicle that allows the electrical storage device to be charged with electric power that is supplied from the outside of the vehicle (so-called plug-in hybrid vehicle). Hereinafter, charging the electrical storage device with electric power that is supplied from the outside of the vehicle is also referred to as “external charging”.
When external charging is performed, the electrical storage device is mostly placed in a state close to a full charge state, so the remaining level of the electrical storage device is high. Therefore, the hybrid vehicle travels in an EV mode (CD mode) in which electric travel is given a higher priority than hybrid travel until the remaining level of the electrical storage device decreases to a mode change threshold after external charging. After that, when the remaining level of the electrical storage device becomes lower than or equal to the mode change threshold, the hybrid vehicle travels in an HV mode (CS mode).
For example, in the EV mode, the start power threshold and the stop power threshold are respectively set so as to be higher than the start power threshold and the stop power threshold in the HV mode. Thus, when the hybrid vehicle travels in the EV mode, a state where the engine is not operated frequently occurs.
On the other hand, the internal combustion engine mounted on the hybrid vehicle, as well as an internal combustion engine mounted on an ordinary vehicle, includes a fuel vapor gas purge system that introduces fuel vapor, produced inside a fuel tank, into an intake passage of the internal combustion engine (that is, the fuel vapor gas purge system purges fuel vapor gas). Fuel vapor gas is purged when the load of the engine is stable, and purging of fuel vapor gas is stopped when a variation in the load is large.
When fuel cut control is ended by changing the accelerator operation amount from zero to non-zero and fuel vapor gas starts to be purged immediately after the end of fuel cut control, the air-fuel ratio of air-fuel mixture that is supplied to the engine significantly fluctuates. In one related art, fuel vapor gas starts to be purged after a lapse of a predetermined period of time from an end of fuel cut control. Furthermore, in the related art, when a predetermined operation state in which fuel cut control is frequently executed through a frequent change of the accelerator operation amount has been detected, the period of time from the end of fuel cut control to the start of purging fuel vapor gas is shortened as compared with that in an ordinary operation state. Because the accelerator operation amount is frequently changed in the predetermined operation state, even when the air-fuel ratio slightly fluctuates due to an early start of purging fuel vapor gas, it is less likely that a driver feels torque fluctuations. As a result, according to the related art, it is possible to reliably purge fuel vapor gas while not making the driver experience torque fluctuations.
Incidentally, there is a driver who drives a vehicle while frequently changing the accelerator operation amount although the driver does not intend to change the vehicle speed by a large amount. That is, although such a driver does not intend to change the vehicle speed by a large amount, the driver frequently changes the accelerator operation amount from zero to non-zero or from non-zero to zero. Such a driving operation is also referred to as “aggressive driving operation (specific driving operation) or aggressive accelerator operation”. When aggressive driving operation is performed, a variation in the load of the engine increases, so purging of fuel vapor gas is frequently stopped. In addition, as described above, in the hybrid vehicle, the operation of the engine is frequently stopped through intermittent operation, so the hybrid vehicle has a smaller number of opportunities to purge fuel vapor gas than the ordinary vehicle that includes only the internal combustion engine as the drive source. From above, when aggressive driving operation is performed in the hybrid vehicle, the amount of purged fuel vapor gas may be insufficient.